1. Technical Field
The present invention relates to an electrophotographic photoreceptor (hereinafter, also referred to as a photoreceptor, simply) used in a copying machine, a printer and the like of electrophotography, an image forming apparatus and an image forming method using the electrophotographic photoreceptor.
2. Description of Related Art
In recent years, image forming in digital form has become mainstream in image forming methods of electrophotography due to development of digital technology. An image forming method in digital form is based on visualization of a pixel image having small pixels such as 400 dpi (dots per inch=2.54 cm). Therefore high image quality technology for re-creating such a small pixel image faithfully is required of the method. In particular, a miniaturized, high resolution and full color copying machine and a printer with improved resolution have been demanded strongly in recent years. A further high image quality technique is required in case of requiring high accuracy, such as high resolution.
Control of the form factor or the particle size distribution of toner and miniaturization of toner particles have been studied for such a demand of enhancing image quality. Development for trying to achieve enhancement of image quality by sharpening form distribution and particle size distribution of toner and miniaturizing toner particles to improve resolution, refined half-toning and the like, is made.
However, enhancing image quality by use of small-particle-size toner is not as effective as initial expectation. Actually, problems are caused by use of small-particle-size toner. That is, one of them is a problem that a toner image formed on a photoreceptor does not have adequate transferability and a problem in cleaning residual toner. The adhesive force of toner to a photoreceptor becomes stronger apparently by miniaturization of toner. As a result the problem that the transferability easily degrades and cleaning becomes difficult, is caused. In particular, a toner image developed with small-particle-size toner having an average particle size of not more than 8 μm has inferior transferability from a photoreceptor to a recording medium and cleanability of residual toner on a photoreceptor, inadequate sharpness, and a strong tendency for poor cleaning caused by toner passing between a photoreceptor and an edge of a cleaning blade, which is so-called “slip through”.
To solve such a problem, techniques that a surface layer of an electrophotographic photoreceptor is caused to include a surface energy reduction agent (fine particles and the like) for reducing surface energy in order to diminish an adhesive force of toner to improve transferability and to diminish a frictional force to the blade, and the like, have been studied. For example, JP-Tokukaihei-5-181291A shows a photosensitive layer is caused to include fine particles of alkylsilsesquioxane resin. However, alkylsilsesquioxane is hygroscopic. Because wettability of the surface of a photoreceptor, that is, surface energy increases under a humid circumstance, a problem, such as easy degradation of transferability, is caused.
JP-Tokukaisho-63-56658A discloses an electrophotographic photoreceptor that is caused to include fluororesin powder for reducing surface energy of the surface of a photoreceptor. However, it was found that uneven halftone was easy to occur when transferability and cleanability were improved by use of fluororesin powder. Furthermore it was found that the phenomenon that unevenness of density was caused in halftone image part had a tendency to be caused easily in cases of an electrophotographic image with a cylindrical photoreceptor. Then, in the discussion of the inventors, it is presumed that the phenomenon is caused since surface energies are not reduced uniformly in the surface of the photoreceptor and variations occur in a contact angle of the photoreceptor when surface energies of the surface are reduced by using a surface energy reduction agent to the surface layer of a cylindrical electrophotographic photoreceptor.